Elevation of transistor channels to reduce impact of shallow trench isolation on transistor performance

ABSTRACT

Roughly described, transistor channel regions are elevated over the level of certain adjacent STI regions. Preferably the STI regions that are transversely adjacent to the diffusion regions are suppressed, as are STI regions that are longitudinally adjacent to N-channel diffusion regions. Preferably STI regions that are longitudinally adjacent to P-channel diffusions are not suppressed; preferably they have an elevation that is at least as high as that of the diffusion regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 11/364,391, filed 27 Feb.2007.

FIELD OF THE INVENTION

The invention relates to methods and systems for improving integratedcircuit performance through stress-engineering of the layout, andarticles manufactured thereby.

Introduction

It has long been known that semiconductor materials such as silicon andgermanium exhibit the piezoelectric effect (mechanical stress-inducedchanges in electrical resistance). See for example C. S. Smith,“Piezoresistance effect in germanium and silicon”, Phys. Rev., vol. 94,pp. 42-49 (1954), incorporated by reference herein. The piezoelectriceffect has formed the basis for certain kinds of pressure sensors andstrain gauges, but only recently has it received attention in themanufacture of integrated circuits. In integrated circuit fabrication,one of the major sources of mechanical stress is the differentialexpansion and contraction of the different materials used. For example,typical fabrication technologies involve electrically isolating theactive regions of groups of one or more transistors by surrounding themwith shallow trench isolation (STI) regions which are etched into thesilicon and then filled with an insulator, such as an oxide. The fillingis performed at an elevated temperature. During the subsequent wafercooling, oxides tend to shrink less than the surrounding silicon, andtherefore develop a state of compressive stress laterally on the siliconregions of the device. Of significance is the stress exerted by the STIregions on the silicon forming a Metal-Oxide-Semiconductor Field-EffectTransistor (MOSFET) channel, because the piezoelectric impact of suchstress can affect carrier mobility, and therefore current flow throughthe channel (Ion). In general, the higher the electron mobility in thechannel, the faster the transistor switching speed.

The stress exerted on a region of silicon decays rapidly as a functionof distance from the stress-causing interfaces. In the past, therefore,while process technologies could not produce today's extremely narrowchannel widths, the stress-induced impact on performance could beignored because only the edges of the diffusion region (adjacent to theSTI regions) were affected. The channel regions were too far away fromthe STI regions to exhibit any significant effect. As processtechnologies have continued to shrink, however, the piezoelectric effecton transistor performance is no longer negligible.

Methods have been developed to model the impact of stress on thebehavior of integrated circuit devices at the level of individualtransistors. These methods include, for example, full-scale analysiswith a Technology Computer Aided Design (TCAD) system; and a methodknown as the “Length-of-Diffusion” (LOD) method described in R. A.Bianchi et al., “Accurate Modeling of Trench Isolation InducedMechanical Stress Effects on MOSFET Electrical Performance,” IEEE IEDMTech. Digest, pp. 117 120 (December 2002), in U.S. Patent PublicationNo. 2002/0173588 (2003), and in Xuemei (Jane) Xi, et al., “BSIM4.3.0Model, Enhancements and Improvements Relative to BSIM4.2.1”, Universityof California at Berkeley (2003), available athttp://www.device.eecs.berkeley.edu/, all incorporated herein byreference. In addition, U.S. patent application Ser. No. 11/291,294,filed Dec. 1, 2005, by inventors Victor Moroz and Dipankar Pramanik,entitled “Analysis of Stress Impact on Transistor Performance”, DocketNo. SYNP 0693-1, incorporated herein by reference, describes anothermethod for stress analysis of integrated circuit layouts.

Behaviors characterized by the various methods for analyzing stressimpact at the level of individual transistors can be used to derivecircuit level parameters (e.g. SPICE parameters) of the device forsubsequent analysis of the circuit at macroscopic levels. Such analysiscan help predict whether the circuit will operate as intended, and withwhat margins, or whether the design or layout needs to be revised. Ifrevision is necessary, it typically involves applying certain generalrules-of-thumb, such as increasing the size of any transistor that,according to the stress analysis, turns out to be weaker than expected.But increasing the transistor size can degrade other performancemeasures, such as power consumption, so a compromise becomes necessary.In addition, the impact of stress on transistor performance is layoutsensitive. Since typical irregularities in an integrated circuit layoutresult in different amount of impact on the performance of differenttransistors across the layout, these kinds of compromises typically mustbe made manually on a transistor-by-transistor basis. Still further, ifautomated place-and-route software is then used to re-layout the revisedcircuit design, the revised layout will differ from the original andshow different stress effects than the original, often completelyupsetting the circuit modifications that were made to accommodate thestress impact of the original layout.

The invention described herein addresses methods and systems forimproving integrated circuit layouts and fabrication processes in orderto better account for stress effects. In some aspects of the invention,dummy features are added to a layout either in order to improveuniformity throughout the layout, or to relax known undesirable stress,or to introduce known desirable stress. These dummy features do notinvolve circuit modification, so no compromise among the abovetransistor performance measures is required. They also for the most partdo not involve another pass through automated place-and-route software,so these layout modifications often can be made without risk that theirbenefit will be upset by the re-layout process. The dummy features caninclude dummy diffusion regions added within STI regions to relaxstress, and dummy trenches added within STI regions either to relax orincrease stress. A trench can relax stress by filling it with astress-neutral material or a tensile strained material. A trench canincrease stress by filling it with a compressive strained material.Preferably dummy diffusion regions and stress relaxation trenches aredisposed longitudinally to at least the channel regions of N-channeltransistors, and transversely to at least the channel regions of bothN-channel and P-channel transistors. Preferably compressive stressenhancement trenches are disposed longitudinally to at least the channelregions of P-channel transistors.

In another aspect, stress relaxation trenches are disposed alongsidepower supply buses, within STI regions separating the power supply busesfrom active diffusion regions.

In another aspect, a fast stress analysis algorithms can be used toapproximate the stress in one or more transistor channels, and layoutrevisions such as the above. The stress can be approximated again, andfurther layout revisions made, and so on iteratively until the stress inthe channel, or the value of one or more performance parameters, aresatisfactory.

In another aspect, a standard cell layout is stress-modified so as toimprove its stress uniformity and/or to better isolate it from stressesinduced by features outside the standard cell layout itself.

In yet another aspect, transistor channel regions are elevated over thelevel of certain adjacent STI regions. Preferably the STI regions thatare transversely adjacent to the diffusion regions are suppressed, asare STI regions that are longitudinally adjacent to N-channel diffusionregions. Preferably STI regions that are longitudinally adjacent toP-channel diffusions are not suppressed; preferably they have anelevation that is at least as high as that of the diffusion regions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to specific embodimentsthereof, and reference will be made to the drawings, in which:

FIG. 1 shows a simplified representation of an illustrative digitalintegrated circuit design flow.

FIG. 2 is a flowchart illustrating portions of steps in FIG. 1 involvedin implementing aspects of the invention.

FIG. 3 illustrates a plan view of a typical layout region of anintegrated circuit design.

FIG. 3A illustrates a cross-section of a chip taken at sight-line A-A asshown in FIG. 3.

FIG. 4 illustrates a larger region of the layout of FIG. 3.

FIGS. 5 and 5A illustrate stress-adjustment modifications to the layoutregion of FIGS. 3 and 3A.

FIG. 6 illustrates stress-adjustment modifications to the layout regionof FIG. 4.

FIGS. 7 and 8 illustrate sample layout region in which trenches havebeen added.

FIG. 9 is a symbolic cross-sectional view of a transistor in which thechannel is elevated above the level of adjacent STI material.

FIG. 10 is a representative plot illustrating the amount of stress nearthe surface of a channel region that is elevated by various amountsrelative to the STI regions.

FIG. 11 illustrates a layout region that includes certain transistorsfrom FIG. 3, as well as others.

FIG. 11A is a cross-sectional view of the layout region of FIG. 11,taken at sight lines A-A.

FIG. 11B is a cross-sectional view of the layout region of FIG. 11,taken at sight lines B-B.

FIGS. 12A, 12B and 12C illustrate fabrication steps that can be used toform the P-channel structures of FIG. 11A.

FIGS. 13A, 13B, 13C and 13D illustrate fabrication steps that can beused to form the N-channel structures of FIG. 11B.

FIG. 14 is a simplified block diagram of a computer system suitable forperforming various steps shown in FIGS. 1 and 2.

FIG. 15 illustrates a plan view of a particular layout region, and arevision of that layout region in an aspect of the invention.

FIG. 16 illustrates the layout region of FIG. 15, revised according to adifferent aspect of the invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

FIG. 1 shows a simplified representation of an illustrative digitalintegrated circuit design flow. At a high level, the process starts withthe product idea (step 100) and is realized in an EDA (Electronic DesignAutomation) software design process (step 110). When the design isfinalized, it can be taped-out (step 140). After tape out, thefabrication process (step 150) and packaging and assembly processes(step 160) occur resulting, ultimately, in finished integrated circuitchips (result 170).

The EDA software design process (step 110) is actually composed of anumber of steps 112-130, shown in linear fashion for simplicity. In anactual integrated circuit design process, the particular design mighthave to go back through steps until certain tests are passed. Similarly,in any actual design process, these steps may occur in different ordersand combinations. This description is therefore provided by way ofcontext and general explanation rather than as a specific, orrecommended, design flow for a particular integrated circuit.

A brief description of the components steps of the EDA software designprocess (step 110) will now be provided.

System Design (Step 112):

The designers describe the functionality that they want to implement,they can perform what-if planning to refine functionality, check costs,etc. Hardware-software architecture partitioning can occur at thisstage. Example EDA software products from Synopsys, Inc. that can beused at this step include Model Architect, Saber, System Studio, andDesignWare® products.

Logic Design and Functional Verification (Step 114):

At this stage, the VHDL or Verilog code for modules in the system iswritten and the design is checked for functional accuracy. Morespecifically, the design is checked to ensure that produces the correctoutputs in response to particular input stimuli. Example EDA softwareproducts from Synopsys, Inc. that can be used at this step include VCS,VERA, DesignWare®, Magellan, Formality, ESP and LEDA products.

Synthesis and Design for Test (Step 116):

Here, the VHDL/Verilog is translated to a netlist. The netlist can beoptimized for the target technology. Additionally, the design andimplementation of tests to permit checking of the finished chip occurs.Example EDA software products from Synopsys, Inc. that can be used atthis step include Design Compiler, Physical Compiler, Test Compiler,Power Compiler, FPGA Compiler, Tetramax, and DesignWare® products.

Netlist Verification (Step 118):

At this step, the netlist is checked for compliance with timingconstraints and for correspondence with the VHDL/Verilog source code.Example EDA software products from Synopsys, Inc. that can be used atthis step include Formality, PrimeTime, and VCS products.

Design Planning (Step 120):

Here, an overall floor plan for the chip is constructed and analyzed fortiming and top-level routing. Example EDA software products fromSynopsys, Inc. that can be used at this step include Astro and ICCompiler products.

Physical Implementation (Step 122):

The placement (positioning of circuit elements) and routing (connectionof the same) occurs at this step. Example EDA software products fromSynopsys, Inc. that can be used at this step include the Astro and ICCompiler products. Certain aspects of the invention herein can takeplace during this step, or just afterwards.

Analysis and Extraction (Step 124):

At this step, the circuit function is verified at a transistor level,this in turn permits what-if refinement. Example EDA software productsfrom Synopsys, Inc. that can be used at this step include AstroRail,PrimeRail, Primetime, and Star RC/XT products. Certain aspects of theinvention can take place during this step as well.

Physical Verification (Step 126):

At this step various checking functions are performed to ensurecorrectness for: manufacturing, electrical issues, lithographic issues,and circuitry. Example EDA software products from Synopsys, Inc. thatcan be used at this step include the Hercules product.

Resolution Enhancement (Step 128):

This step involves geometric manipulations of the layout to improvemanufacturability of the design. Example EDA software products fromSynopsys, Inc. that can be used at this step include Proteus, ProteusAF,and PSMGen products.

Mask Data Preparation (Step 130):

This step provides the “tape-out” data for production of masks forlithographic use to produce finished chips. Example EDA softwareproducts from Synopsys, Inc. that can be used at this step include theCATS(R) family of products.

FIG. 2 is a flowchart illustrating portions of steps 122 and 124(FIG. 1) involved in implementing aspects of the invention. As with allflowcharts herein, it will be appreciated that many of the steps in FIG.2 can be combined, performed in parallel or performed in a differentsequence without affecting the functions achieved. In step 210,corresponding roughly to steps 100 and 112-120 in FIG. 1, the designerspecifies a circuit design. As used herein, an “integrated circuitdesign” is a transistor level design, after synthesis from VHDL andbefore layout. A designer can “specify” an integrated circuit designeither by specifying it at the transistor level, or by specifying at ahigher level and manually or automatically converting it to thetransistor level through one or more sub-steps.

For purposes of some aspects of the invention, in the integrated circuitdesign in step 210, the designer has already specified (again,explicitly or implicitly) the channel length/width (L/W) ratios desiredfor each transistor. Since in a typical integrated circuit technologythe channel lengths of most transistors are the same, effectively thespecification of L/W ratios is also a specification of the ratio thateach transistor's channel width bears to every other transistor'schannel width. Many factors are considered in the selection of L/Wratios, one of which is the minimum required current-carrying capacityIon of the transistor in the ON state. In particular, it is known that,absent stress considerations, the Ion of a transistor is roughlyproportional to its channel width. That is, the ratio of Ion of onetransistor to that of a second transistor is roughly equal to the ratioof their channel widths. So if one transistor is required to supplytwice the current for downstream circuitry that a second transistor isrequired to supply, then the first transistor would be assigned twicethe channel width as the second transistor. Based on this principle, onecan determine from the relative channel widths assigned to differenttransistors in an integrated circuit design, the ratios of Ion's thatwere intended by the designer for such transistors.

In step 212, the circuit design undergoes “placement”, and optionally“routing” as well, thereby resulting in a “layout”. Step 212 correspondsroughly to part of step 122 (FIG. 1). As used herein, a “layout” definesa set of masks that, when applied in a fabrication process, togetherdefine the physical features of the integrated circuit device. Amongother things, these features can include transistor source, drain andchannel regions, and diffusion regions, and STI regions, and so on, andtogether these features define circuit structures such as thetransistors specified in the integrated circuit design. The masksdefined by a “layout”, as that term is used herein, may (and typicallydo) go through one or more post-processing steps such as steps 126-130(FIG. 1) before they are finalized for production. Although a layouttypically defines masks for all of the fabrication process steps, itwill be appreciated that for some aspects of the present invention theintegrated circuit design need only be compiled to the point of a layoutthat defines fewer than all such masks. For example, for some aspectsthe layout need not yet define masks for the so-called “back-end”fabrication steps, such as the formation of routing and via layers.

In step 214, in one aspect of the invention, certain stress-relatedenhancements can be added to the layout without specific knowledge ofthe circuitry or device structures. These are enhancements that improvecircuit performance or improve uniformity in most layouts.

Some of the circuit non-specific stress-related enhancements are basedon the understanding that compressive stress exerted on transistorchannel regions enhances performance in certain kinds of situations, anddegrades transistor performance in other kinds of situations. Forexample, compressive stress exerted longitudinally or transversely onthe channel of an N-channel transistor can degrade certain performanceparameters of the transistor. Specifically, electron and hole mobility,and therefore Ion and transistor switching speed, often can degrade byas much as 20-30%/GPa. Layout enhancements that reduce or relievecompressive stress in the channels of N-channel transistors, therefore,are very likely to enhance the performance of these transistors. Asanother example, compressive stress exerted transversely across thechannel of a P-channel transistor often can degrade electron and holemobility, and therefore Ion and transistor switching speed, by as muchas 70%/GPa. On the other hand, compressive stress exerted longitudinallyon the channel of a P-channel transistor often can enhance holemobility, and therefore Ion and transistor switching speed, by as muchas 90%/GPa. Layout enhancements that reduce or relieve transversecompressive stress in the channel of P-channel transistors, and layoutenhancements that increase longitudinal compressive stress in thechannels of P-channel transistors, therefore, are all very likely toenhance the performance of these transistors.

FIG. 3 illustrates a plan view of a typical layout region 300 of anintegrated circuit design. FIG. 3A illustrates a cross-section of theresulting chip taken at sight-line A-A as shown in FIG. 3. Shown in FIG.3 are two P-channel transistors 310 and 312, and two N-channeltransistors 314 and 316. The two P-channel transistors share a diffusionregion 318, and the two N-channel transistors share a differentdiffusion region 320. Each transistor has a channel which is defined byits diffusion region and a gate conductor which crosses the diffusionregion. The drain and source regions of each of the transistors are theportions of the diffusion regions on opposite sides of the gateconductor, but whether one constitutes the source and the other thedrain or vice-versa, depends on the circuit being implemented.

In typical CMOS fashion, to form a logical inverter element, the gateconductor crosses both a P-diffusion and an N-diffusion to define both aP-channel and an N-channel transistor. Thus in FIG. 3 a gate conductor322 crosses both diffusion regions to define both transistors 310 and314, and a gate conductor 324 crosses both diffusion regions to defineboth transistors 312 and 316. The channels of the transistors in atypical fabrication process are slightly different (from left-to-rightin the figure) than the gate conductors themselves because of theaddition of other components of the gate stack (not shown) such asspacers, and lateral diffusion of the source and drain dopants under thegate. As used herein, the term “region” represents a two-dimensionalarea in a plan view of the layout. Stress “in” a region is considered tobe the stress close to the surface of the region, where current flows.In the embodiments described herein, an approximation is made that thestress “in” a region is equal to the stress “at” the surface of theregion. In another embodiment, stresses within a volume of the chip canbe taken into account as well, including at depths below the surface.

As used herein and as shown in FIG. 3, the “longitudinal” direction of atransistor is the direction of current flow between source and drainwhen the transistor is turned on. The “transverse” direction isperpendicular to the longitudinal direction, and perpendicular to thedirection of current flow. Both the longitudinal and transversedirections of the transistor are considered to be “lateral” directions,meaning a direction that is parallel to the surface. Other “lateral”directions include those (not shown) which are parallel to the surfacebut intersect both the transverse and longitudinal directions at angles.The “vertical” direction is normal to the surface of the channel andtherefore perpendicular to all possible lateral directions. The “length”of a structure in the layout is its length in the longitudinaldirection, and its “width” is its width in the transverse direction. Itcan be seen from the layout of FIG. 3 that the channel lengths aresignificantly shorter than their widths, which is typical for thetransistors that are used in logic circuits. Also shown in FIGS. 3 and3A are X, Y and Z coordinate axes of the layout. Primarily forlithographic reasons, it is common in logic circuit design that alltransistors be oriented alike, and consistent with this convention, inthe layout of FIG. 3, all four transistors are oriented such that thelongitudinal direction of the transistors are in the X direction of thelayout, and the transverse direction of the transistors are in the Ydirection of the layout. The Z direction, visible in FIG. 3A, isperpendicular to both the X and Y directions, representing a depth intothe integrated circuit chip.

Additionally, the term “region”, as used herein, does not necessarilyimply a physical boundary. That is, one “region” can contain multiple“sub-regions”, which themselves are considered herein to be “regions” aswell. Thus it is reasonable to refer to a region within a diffusionregion, even one that has not been defined physically in any way. InFIG. 3A, more than one set of source and drain diffusions share a singleoverall diffusion region. In yet another embodiment, the source, drainand channel regions collectively are laterally co-extensive with theoverall diffusion region. Also, in another embodiment, the source anddrain diffusion regions might be made of different materials (e.g. SiGe)than the channel region (e.g. Si). In all of these cases it can be saidthat the source diffusion region forms “at least part of” a diffusionregion, that the drain diffusion region forms “at least part of” adiffusion region, and that a channel region can exist even before it isdefined physically.

FIG. 3 also illustrates power and ground diffusion buses 326 and 328,respectively. Typically metal rails overly these diffusion buses, andsince the present discussion is concerned primarily with plan views oflayout features, it makes little difference whether what is referred tois the diffusion bus or the metal rail. For convenience, therefore, bothbuses and rails are referred to herein simply as “conductors”.

As can be seen from FIG. 3, the transistors and their diffusions aredisposed laterally between the power and ground conductors. The powerand ground conductors, as well as power supply conductors of any othervoltage, are all sometimes referred to herein collectively as “powersupply conductors”.

FIG. 4 illustrates a much larger region of the layout of FIG. 3. Asshown in FIG. 4, this layout includes power supply rails (conductors)that extend across most or all of the chip in the X dimension. Such anarrangement is common, especially but not exclusively for ASICs,standard cells and FPGAs. The power supply conductors 326 and 328 (FIG.3) are shown also in FIG. 4. In typical 2-voltage circuits (power andground), the rails alternate power and ground in the Y dimension. Thetransistors of the logic circuitry are laid out in a strip between apair of the rails, usually within individual cells or macrocells such as410, 412 and 414 in FIG. 4. Typically the cells are all of the same sizein the Y dimension but may vary in size in the X dimension. Cell 412,for example, contains the four transistors with two diffusion regions asshown in FIG. 3. Cell 414 in FIG. 4 represents diffusion regions as thesmaller rectangles, and the regions between the rails and outside of thediffusion regions are STI regions containing oxide. All these STIregions conventionally exert compressive stress on the diffusionregions, including within the transistor channels, both longitudinallyand transversely.

Returning to FIG. 3, arrows have been inserted to illustrates variouscomponents of the STI-induced compressive stress exerted on the fourchannel regions. It can be seen that stress is exerted bothlongitudinally and transversely. Some of the transverse stresscomponents are also shown in FIG. 3A. As mentioned above, all suchcompressive stress components tend to degrade performance of thetransistors except for longitudinal stresses on P-channel transistors310 and 312, which tend to enhance performance. The performanceenhancing stress components are indicated in FIG. 3 by darkened arrows.In particular, it is noteworthy that all transverse stress componentsare detrimental.

As mentioned above, in step 214 (FIG. 2), in one aspect of theinvention, certain stress-related enhancements can be added to thelayout without specific knowledge of the circuitry or device structures.Because all transverse stress components tend to degrade transistorperformance, one such technique is to insert longitudinally-orientedstress relief trenches in the STI regions, spaced laterally from thetransistor channels. This step can be taken based on specific knowledgeof the actual locations of the device structures (as described in moredetail hereinafter), but can also be taken without knowledge of thelocations of device structures in circuits laid out according to theconventions shown in FIG. 4. That is, in a layout in which mosttransistors are located laterally in strips between power supply rails,and most transistors are oriented so that their longitudinal directionlies in the same direction as the orientation of the power supply rails,transverse stress can be relieved for most transistors in the layout byintroducing a trench in the STI region laterally between the transistordiffusion regions and either or both of the power supply conductors,extending parallel to the power supply conductors.

FIGS. 5 and 5A illustrate such a modification to the layout region ofFIGS. 3 and 3A. FIG. 6 illustrates the modification to the region ofFIG. 4. As can be seen, a trench 510 has been inserted into the layoutwithin the STI region 330, oriented in the X dimension, and disposedtransversely between the diffusion region 318 and the power supplyconductor 326. Similarly, another trench 512 has been inserted into thelayout within the STI region 334, oriented in the X dimension, anddisposed transversely between the diffusion region 320 and the powersupply conductor 328. The transverse component of the STI-induced stresshas been relieved, as indicated by the absence of those arrows in theFigure. Not all transverse stress is relieved, but the improvement cannevertheless be significant and the technique can be applied as a matterof course, without specific knowledge of the circuit design or thelayout. Moreover, since the trenches do not alter the integrated circuitdesign in any way, there is no need to pass the design through softwarefor a new layout.

The trenches 510 and 512 can be etched at the end of the front-endfabrication process, after all the high temperature steps are completed.The trenches can be of any width (in the X dimension), since their onlypurpose is to relieve stress. Preferably they are made as narrow as thefabrication technology will allow. They are also preferably as deep aspossible without, however, breaking through the STI into the siliconbelow. However, having its depth as small as ⅔ or even ⅓ of the STIdepth is often sufficient to relax most of the harmful stress. It isalso easy to extend the trenches along the full length of the powersupply lines as shown in FIG. 6. However, since the beneficial effect ofthe trenches is felt most substantially (though not exclusively) fromonly those portions directly alongside a diffusion region such as 318 or320, it is still possible to obtain significant performance enhancementby implementing stress relief trench segments, only alongside one ormore of the diffusion regions. Significant performance enhancement canalso be obtained even if the trenches are limited to segments disposeddirectly transverse to the channel regions. The layout will benefit fromthis technique also if trenches are disposed along only one of the twopower supply conductors bordering a strip of cells, although trenchesalong both power supply conductors are preferred.

The descriptions herein of the stress impact on transistors and of themethods to use stress to improve transistor performance apply to whatare presently the standard crystallographic orientations used in thesemiconductor industry, with the (100) wafer surface and <110> channeldirection. The stress distribution changes only slightly for alternativepossible crystal orientations of the wafer and the transistor channel,but the impact of stress on carrier mobility can change significantlynot only in magnitude, but also in sign. Therefore, the describedmethodologies can be still applied to an arbitrary crystal orientationof the wafer and the transistors, but the type and location of thestress-improving trenches, dummy features and other techniques will needto be adjusted for each specific case. The same is true for alternativesemiconductors like germanium and compound semiconductors like GaAs,InP, SiC.

In one embodiment, the STI material can be formed as an oxide (forexample a thermally grown oxide) on the walls and bottom surfaces of theSTI trenches, a nitride liner formed on the thermal oxide, and a secondoxide (for example TEOS) filling the remainder of the trenches above thenitride liner. The second oxide can be then etched to form thestress-reduction trenches, with the nitride liner ensuring that thetrenches do not go deeper than the STI.

The stress relief trenches 510 and 512 may be filled with any material,but preferably a dielectric material rather than a conductor. Also thefill material should not be one that will not re-introduce the stressthat the trench was introduced in order to relieve. Preferably a lowtemperature fill, such as TEOS deposited at low temperature, is used inorder to avoid new stresses created by the downward temperature ramp.Another suitable fill material is a low-k dielectric such as that usedfor interconnects. For both low temperature TEOS and low-k dielectrics,use of these materials to fill stress relief trenches does not requireadditional process steps because these materials are deposited anywaybefore application of the first metal layer. Yet another satisfactoryfill material is a nitride which has little thermal mismatch withsilicon.

Even more preferably, however, the trenches are filled with a materialthat introduces tensile, as opposed to compressive stress in thetransverse dimension. For example, a strained material such ascommercially available strained silicon nitride can be used. For narrowtrenches, such a material can be deposited over the entire wafer,thereby filling the trenches and leaving a thin layer over the rest ofthe wafer. The silicon nitride material outside the trenches can then beremoved by a wet etch, or by a dry etch with the trench regions masked,or by chemical-mechanical-polishing (CMP), or by other methods that willbe apparent to the reader. The extra process steps required by thefilling of the trenches with a stained material may be most justifiablefor high performance or high margin integrated circuit products.Alternatively, in fabrication processes that already use tensile nitrideas cap overlayer to boost performance of nMOSFETs, the material can bedeposited in the trenches during the same process step.

Returning to FIG. 2, in addition to layout revisions to incorporate anycircuit insensitive stress enhancement techniques, a series of stepsalso can be taken which apply stress-enhancement techniques that dodepend on the circuit design and current layout. In general, theapplication of these techniques take the overall form of a step 216, inwhich the stress impact on a transistor performance parameter isanalyzed, followed by a step 218, in which it is determined whether thestress-adjusted performance parameter matches the target value for thatparameter. If not, then in step 220 the layout is revised according toone or more stress-enhancement techniques, and the method loops back tostep 216 for re-analysis. The sequence of steps can be performediteratively until the value(s) of one or more performance parameters aredeemed satisfactory.

The stress analysis can be performed by any desired method, includingfull TCAD simulation. The LOD method can be used, but is not preferredbecause of its inherent inaccuracies. Most preferred is the methoddescribed in the above-incorporated “Analysis of Stress Impact onTransistor Performance” Patent Application, because it can be made tooperate with sufficient speed to analyze the transistors in large layoutregions in multiple layout revision iterations, and with sufficientaccuracy. Roughly described, that method involves, for each transistorto be analyzed, first selecting several sample points in thetransistor's channel. The stress vector at each of the sample points isthen approximated, and the impact on a transistor characteristic ofinterest, such as the stress-induced change in mobility at theparticular sample point, is determined. The values of thesecharacteristics are then averaged over all the sample points in thechannel to approximate the average stress-adjusted value for the entirechannel.

The transistor performance parameter referred to in steps 216 and 218 isany parameter of a transistor that can then be used in circuit levelsimulations, such as SPICE. Examples include electron mobility, Ion, andtransistor switching speed. In addition, as used herein, a “parameter”is considered merely a slot or container. It is not itself a value.However, in a particular circuit or structure, a parameter can have avalue. The present discussion refers to such a value as the “value” ofthe particular parameter.

In step 220, a number of different techniques are available to revisethe layout to account for stress modification of the performanceparameter. In one aspect, the transistor channel widths can be adjustedto better match the strength ratios intended by the designer. Moreparticularly, referring to FIG. 3, it can be seen that the channel widthof transistor 312 is approximately 3 times the channel width oftransistor 310, and similarly the channel width of transistor 316 isapproximately 3 times the channel width of transistor 314 (drawings arenot to scale). Since transistor strength (Ion) is roughly proportionalto channel width/length (absent stress considerations), and since thechannel lengths are all the same (as is typically the case), it appearsthat the designer intended the Ion for transistor 312 to be 3 times thatof transistor 310. Similarly, it appears that the designer intended theIon for transistor 316 to be 3 times that of transistor 314. Thisstrength ratio of 3:1 is referred to herein as a target ratio, andbefore stress effects are taken into account, results in the 3:1 channelwidth ratio on the layout. The analysis in step 216, however, mayindicate that actual strength ratio is significantly different than 3:1once stress effects are considered. In step 220, therefore, the designercan either increase the channel width (i.e. the diffusion width) of oneof the transistors, or decrease the channel width of the other, or acombination of both, in order to achieve the target strength ratio of3:1. The method then returns to step 216 to re-analyze thestress-adjusted ratio of the values of Ion. Further layout revisions maybe performed thereafter, in iterative fashion until the Ion ratios, aswell as the values of any other transistor performance parameters ofinterest, are within acceptable ranges of their target values.

Note that often it will be insufficient to modify the channel widths ofonly the particular transistors being addressed. Increased channel widthalso causes increased capacitance, often requiring upstream drivingcircuitry to be strengthened to accommodate. On the other hand,decreased channel width weakens the driving ability of the transistor,which may require adjustment of downstream circuitry. Therefore, afterthe layout revisions are made, it is advisable to calculate new SPICEmodel parameters for the affected transistors and re-run the circuitsimulations to ensure that the circuit still will operate as intended.

A second technique that can be used to revise the layout to account foror counteract stress modification of a performance parameter in step220, involves introducing dummy features at strategic locations in thelayout. These dummy features are not electrically connected to thecircuitry, thereby avoiding any necessity to re-layout the design afterstress-related layout revision. In one embodiment, the dummy featuresare trenches, optionally filled with compressive or tensile strainedmaterial as required either to reduce undesirable stress or increasedesired stress. FIG. 7 illustrates one sample layout region in whichsuch trenches have been added. The layout defines a P-channel transistor710 and an N-channel transistor 712 sharing a common gate conductor 714.Transistor 710 has a P-channel diffusion 716 and the transistor 712 hasan N-channel diffusion 718. As previously pointed out, compressivestress degrades N-channel transistor performance regardless of thedirection. Therefore, the layout of FIG. 7 has been modified by adding atrench 720 completely surrounding the N-channel diffusion 718. Byitself, this trench should reduce stress (at least STI-induced stress)on the diffusion region 718 and thereby improve transistor performance.In addition, in the embodiment depicted in FIG. 7, the trench 720 hasbeen filled with a tensile strained material such as tensile nitride. Asa result, stress in the diffusion region 718 is affirmatively reduced,even to the point of being tensile, thereby improving performance of thetransistor even further.

With respect to the P-channel transistor 710, as previously pointed out,compressive stress in the transverse direction degrades P-channeltransistor performance but compressive stress in the longitudinaldirection improves P-channel transistor performance. Therefore, twotrenches 722 and 724 have been added to the layout of FIG. 7, orientedlongitudinally and spaced transversely on either side of P-channeldiffusion region 716. These trenches have been filled with a tensilestrained material to further apply tensile stress on the diffusionregion 716 transversely. In addition, two more trenches 726 and 728 havebeen added to the layout of FIG. 7, oriented transversely and spacedlongitudinally on either side of P-channel diffusion region 716. Thesetrenches have been filled with a compressive strained material such ascompressive nitride or compressive TEOS, to further apply compressivestress on the diffusion region 716 transversely. Both of these layoutmodifications tend to improve performance of transistor 710.

Note that in an embodiment, the layout modifications of FIG. 7 can beperformed as a matter of course for all (or many) N-channel andP-channel transistors in a layout, outside of the iteration loop ofsteps 216, 218 and 220 if desired. Also, in an embodiment,stress-adjustment trenches, whether or not filled with tensile orcompressive strained material, can be given varying depths. Control oftrench depth adds additional designer flexibility because deepertrenches tend to affect stress over greater lateral distances, whereasthe stress effects of shallower trenches tend to be more localized.Designer flexibility can be enhanced also by using two or more differentfill materials having different strains in different trenches. Forexample, fill materials that can be used include commonly availablepre-strained nitride with various compressive strains of up to −2.5 GPa,and with various tensile strains of up to +1.5 GPa. Thus, for example,if certain regions of a layout are determined to be under undesirablehigh tensile stress, stress can be made more neutral in these regions byadding appropriately disposed deep trenches filled with −2.5 GPacompressive nitride. If certain regions of a layout are determined to beunder low but still undesirable tensile stress, stress can be made moreneutral in these regions by adding appropriately disposed shallowtrenches filled with compressive TEOS. Similarly, if certain regions ofa layout are determined to be under undesirable high compressive stress,stress can be made more neutral in these regions by adding appropriatelydisposed deep trenches filled with +1.5 GPa tensile nitride; and ifcertain regions of a layout are determined to be under low but stillundesirable compressive stress, stress can be made more neutral in theseregions by adding appropriately disposed shallow trenches filled with0.5 GPa tensile nitride. Many other variations will be apparent.

FIG. 8 illustrates that the trenches added to a layout in order tomodify it for stress considerations need not be simple rectangles as inFIGS. 5, 6 and 7. In FIG. 8, the trench is shaped in a mannercomplementary to the target diffusion region, to improve its effectfurther. In particular, a diffusion region 810 contains two transistors812 and 814, defined by the diffusion region in conjunction with gateconductors 816 and 818, respectively. Transistor 814 has a largerchannel width than transistor 812, so the diffusion region transitionsfrom a narrower width to a wider width at an edge 820, at a positionthat is longitudinally between the two channel regions. A trench 822 hasbeen added to the layout, oriented longitudinally and spacedtransversely from the diffusion region 810. In the embodiment, thetrench 822 has been filled with a tensile strained material. The trenchis wider adjacent to the narrower transistor 812 than it is adjacent tothe wider transistor 814, and it transitions from its wider width W1 toits narrower width W2 at an edge 824 of the trench 822. In this way,trench 822 has approximately the same lateral spacing from the channelregions of both transistors 812 and 814, whereas a simple rectangulartrench might have to be spaced farther from the channel of transistor812 than from the channel of transistor 814. A narrower spacing betweenthe trench 822 and the channel of transistor 812 tends to improve thebeneficial effect of the trench, since stress falls off as a function ofdistance. The wider trench near the channel of transistor 812 also helpsimprove the performance of transistor 812 since the greater transversewidth of tensile strained fill material imposes greater tensile stresson such channel.

In the layout of FIG. 8, the longitudinal dimension L2 of the trench 822should preferably be as long as possible. However, since the mostbenefit derives from portions of the trench directly transversely spacedfrom each channel region, one embodiment includes such a trench onlydirectly transversely spaced from each channel region. In addition, itcan be seen that in the layout of FIG. 8, the trench 822 extendstransversely into the cutout left by the diffusion region 810 when ittransitions from the wider channel width to the narrower channel widthat edge 820, so that the trench 822 and the diffusion region 810 overlapin the transverse dimension by a distance indicated ‘OV’ in the drawing.At least the overlapping portion of the tensile filled trench 822 willtherefore apply tensile stress longitudinally on at least theoverlapping portion of the diffusion region 810. For N-channeltransistors, this is beneficial, so OV should be made large if possible.For P-channel transistors this is detrimental, so OV should be keptsmall if possible. Regardless of the transistor type, it will beadvantageous to optimize the positions and shapes of the trenches byiterative “what-if” evaluation of various configurations in the loop ofsteps 216, 218 and 220 (FIG. 2). The depth of the trenches and the fillmaterial can also be optimized during these iterations, based on thegeneral rules described above.

It can be seen that introducing dummy trenches at strategic locationsand with strategic shapes and fills in the layout during the iterativelayout revisions of steps 216, 218 and 220 can improve performance ofthe transistors targeted by these techniques. One or more additionallithography steps may be incurred in order to implement this aspect ofthe invention. However, the additional lithography steps can be avoidedin fabrication technologies that include the use of silicon germaniumsource and drain regions for P-channel transistors. Silicon germanium isa more compressively strained material than STI, so trenches orientedtransversely and spaced longitudinally from the ends of P-channeldiffusions, such as in regions 726 and 728 of FIG. 7, can be filled withsilicon germanium in order to introduce compressive stresslongitudinally into the channel. Similarly, the additional lithographysteps can be avoided also in fabrication technologies that use carbondoped silicon, which is a tensile strained material. Thus carbon dopedsilicon may be used to fill trenches such as 722, 724, 720 and 822 inFIGS. 7 and 8, often without incurring the cost of additionallithographic steps. Note that both silicon germanium and carbon dopedsilicon are electrically conductive, whereas strained silicon nitride isnot. Unlike trenches filled with strained silicon nitride, therefore,the locations of trenches filled with silicon germanium or carbon dopedsilicon should be taken into account when routing the interconnects.

In a third technique for revising the layout to account for stressmodification of a performance parameter, dummy diffusion regions areadded in the STI regions of the layout instead of or additionally to theadding of dummy trenches. Many of the same principles apply to thelocations and shapes of such dummy diffusion regions as set forth abovewith respect to the locations and shapes of stress relief trenchesfilled with stress-neutral material. The use of dummy diffusion regionsmay not be as flexible as the use of trenches filled with strainedmaterials, but no additional process steps are required. These dummydiffusions preferably are placed as close as possible to the transistordiffusions in order to maximize their effectiveness for stressreduction. Preferably they are spaced from a transistor diffusion by nomore than one or two times the minimum STI width specified for thefabrication process.

In yet a fourth technique for revising the layout to account for stressmodification of a performance parameter, diffusion regions containingmore than one P-channel transistor longitudinally can be split betweentransistors. This has the effect of introducing an STI regionlongitudinally between the two diffusions, thereby introducingbeneficial compressive stress into the channel regions of both.Typically it is not desirable to split shared diffusion regions as amatter of course, since this technique usually increases the amount ofchip area required to implement the circuit. But the tradeoff may beworthwhile for selected transistors, such as those in a critical path,or it may be worthwhile for most or all P-channel transistors in highperformance or high margin products.

Yet a fifth technique for revising the layout to account for stressmodification of a performance parameter is illustrated in FIG. 15. FIG.15 illustrates a layout region 1510 having five transistors representedby channel regions 1512, 1514 a, 1514 b, 1514 c and 1514 d, all in asingle diffusion 1516. The transistor 1512 has a channel width w1 and islocated in a wider segment of the diffusion region 1510 than thetransistors 1514 a, 1514 b, 1514 c and 1514 d (collectively 1514), whichall have a channel width w2. The diffusion 1516 transitions from thewider width to the narrower width at top and bottom transition edge 1518and 1520, respectively.

According to this fifth technique, the lengths of diffusion regions canbe changed in a longitudinal direction without changing the widths.Typically these changes will involve extending rather than contractingthe diffusion lengths, in a direction away from the channel regions.Thus as illustrated in FIG. 15, the left edge of diffusion region 1516is extended toward the left (as illustrated by the dashed lines and anarrow pointing in the direction of extension), and the right edge ofdiffusion region 1516 is extended toward the right. Additionally, thetransition edges 1518 and 1520 are moved toward the right, which is awayfrom the transistor 1512 that they affect, but not so far toward theright so as to enlarge the channel width of the left-most narrowtransistor 1514 a.

If the diffusion region 1516 is entirely silicon, then extending itlongitudinally away from the channel regions has the effect of reducingSTI-induced compressive longitudinal stress on the channel regions. Thisis beneficial for N-channel transistors only, and therefore in anembodiment, the technique is used only on N-channel diffusions. If thediffusion region 1516 contains SiGe in the source and drain portions, orcontains another material that causes compressive stress longitudinallyinto the transistor channels, then extending the regions longitudinallyaway from the channel regions has the effect of increasing compressivelongitudinal stress on the channel regions. This is beneficial forP-channel transistors only, and therefore in an embodiment, thetechnique is used only on P-channel diffusions containing acompressively stressed material in the source and drain regions.Commonly, however, N-channel diffusions on a chip are made of siliconwhile P-channel diffusions on the same chip contain SiGe source anddrain regions. In this situation extending the diffusion regionslongitudinally away from the transistor channel regions would bebeneficial for both the N-channel and the P-channel transistors.

A layout revision according to this fifth technique does not requireiteration back to the circuit design, since so long as the channelwidths remain unchanged, so do the load capacitances.

Yet a sixth technique for revising the layout to account for stressmodification of a performance parameter is illustrated in FIG. 16. FIG.16 illustrates the layout region 1510 of FIG. 15, revised according tothis sixth technique. In this sixth technique, all transistors are giventhe same channel width by replacing wider transistors with anappropriate number of parallel-connected narrower transistors. In thelayout region of FIG. 15, for example, transistor 1512 has twice thewidth as each of the transistors 1514 (w1=2*w2). In the revised layoutof FIG. 16, therefore, transistor 1512 has been replaced by twoparallel-connected transistors 1610 and 1612, each having width w2. Thetwo replacement transistors 1610 and 1612 are connected in parallel byconnecting the gate of transistor 1610 to the gate of transistor 1612and the source of transistor 1610 to the source of transistor 1612, asshown symbolically in FIG. 15 by interconnects 1614 and 1616, and bypositioning them to share a common drain. Other means ofparallel-connecting the replacement transistors will be apparent to thereader.

It will be appreciated that since transistor 1512 was designed withtwice the width as transistors 1514, the designer appears to haveintended that transistor 1512 have twice the Ion as each of thetransistors 1514. Because of stress effects, however, that designerintent will not be fulfilled as originally laid out. By replacing thetransistor 1512 with two parallel-connected transistors of half thewidth as transistor 1512, and of the same width as transistors 1514, thelayout revision yields a combined Ion of the replacement transistorsequal to twice the Ion of each of the transistors 1514, as apparentlyintended by the designer.

If the layout contains two transistors having widths that are notinteger multiples of each other, this sixth technique can still beapplied if both transistors are replaced by a respective set ofparallel-connected transistors. The intended ratio of Ions will beachieved if all the replacement transistors have the same width, and ifthe ratio of the number of transistors replacing the first transistor tothe number of transistors replacing the second transistor is equal tothe ratio of the channel width of the first transistor as originallylaid out, to the channel width of the second transistor as originallylaid out. For example, if transistor A and transistor B are originallylaid out with widths in the ratio of 3:2, then transistor A can bereplaced by 3 parallel-connected transistors and transistor B can bereplaced by 2 parallel-connected transistors, all of the same width. Thetechnique can easily be extended to include replacement of multipletransistors as originally laid out, with multiple corresponding sets ofparallel-connected narrower transistors.

It will be appreciated also that the revision of transistor widths suchthat all (or most) transistors in the overall layout have the samewidth, can substantially improve stress uniformity and layoutinsensitivity. It therefore may be desirable to revise layouts accordingto this sixth technique whenever possible, even without simulatingactual stress-induced variations caused by different designed transistorwidths.

In addition to the six techniques described above, it will beappreciated that other techniques can also be used to revise the layoutin step 220 in response to stress effects. In addition, several of thetechniques are compatible with each other, such that more than one ofthem can be used in a single layout or layout region.

After one or more such techniques are applied in the loop of steps 216,218 and 220, and the stress-adjusted values of all target transistorperformance parameters are satisfactory, the user can proceed tosubsequent steps of the EDA process such as analysis and extraction step124, and so forth (step 222). As used herein, a layout revised forstress effects is sometimes referred to herein as having been formed “independence upon” the automatically-generated layout from step 212. Asused herein, a given layout is formed “in dependence upon” a predecessorlayout if the predecessor layout influenced the given layout. If thereis an intervening step or time period, or if there are other stepsperformed between the step 212 layout and the given layout, the givenlayout can still be “in dependence upon” the predecessor layout. If theintervening step combines more than one layout, the given layout isconsidered to have been formed “in dependence upon” each of thepredecessor layouts.

At this point it should be noted that the introduction of stressmodifications into a layout can be beneficial even where transistorperformance is not improved, because a benefit can be obtained merely byremoving or reducing the sensitivity that transistor performanceotherwise has to its positions and surroundings in a particular layout.For example, if a minor change in the circuit design causes a particulartransistor to be positioned differently in the resulting layout, and ifthe stress effects then cause the transistor's Ion value to depend onits position and its neighborhood in the layout, then the minor changein the circuit design might produce unintended results after the layoutstep. This can require the designer to revisit upstream steps in the EDAprocess of FIG. 1 to correct the unintended consequences. Thecorrections then applied in the circuit design might again produceunintended consequences after layout, requiring the designer to changethe circuit design yet again, and so on. A stress modification step thatreduces the sensitivity of transistor performance to its position in thelayout, therefore, can be beneficial by helping to isolate the circuitdesign step from the layout step, thereby reducing the need to revisitupstream EDA steps. Accordingly, in another embodiment, decision step218 in FIG. 2 can be replaced by a decision step asking whether thestress-modified layout removes a layout-induced variation in transistorperformance.

The removal of layout sensitivity is beneficial especially in thecontext of standard cells, because layout-dependent stress can causetiming variations from instance to instance of the same cell dependingon the cell placement and its neighborhood. A premise of standard celldesign is that optimally the same cell design and layout can be used andre-used, wherever desired and without adjustments made internally toaccount for the context of its use. Some or all of the above techniquescan therefore be used in standard cell layouts in order to isolate thecell from external stress influences. In particular, for example, dummydiffusions or trenches can be added along cell boundaries and/or alongpower supply conductors to reduce stress interaction of internaltransistors from outside stress sources. Some dummy diffusion structurescan be formed simply as extensions of well taps. Others can be turnedinto antenna diodes, effectively re-using chip area otherwise used foran external antenna diode. Dummy diffusions and trenches disposedalongside power supply conductors as shown in FIGS. 5 and 6 can alsohelp isolate the cell from external stress influences.

Yet another stress management technique involves elevation of thetransistor channel regions above the level of the adjacent STI material,as shown symbolically in FIG. 9. As shown in FIG. 9, a device includes asilicon substrate 910, into which a diffusion well 912 has been formed.STI regions 914 and 916 bracket the well 912 in at least the transversedimension, as that dimension is defined by one or more transistorchannel regions not shown in FIG. 9 but within the diffusion 912. Thusin FIG. 9, the longitudinal direction extends perpendicularly to thepage. It can be seen that the diffusion region in FIG. 9 has beenelevated relative to the STI transversely-adjacent regions. Statedanother way, the transversely-adjacent STI regions have been suppressedrelative to the diffusion region. The elevation differential causes thecompressive stress induced by the STI regions on the diffusion region tobe felt mostly at some depth below the surface of the diffusion region,allowing the surface of the diffusion region, where most of the currentflows, to remain relaxed. This stress-management technique can be usedeither instead of or in addition to other techniques set forth herein,but when used, other techniques are likely to be less effective and lessuseful since undesirable STI-induced stress is already reduced by theelevation differential.

FIG. 10 is a representative plot illustrating the amount of stress nearthe surface of a channel region that is elevated by various amountsrelative to the STI regions. It can be seen that if the stress in thechannel with no elevation is 500 MPa, then the stress in the channelwith an elevation of 50 nm is reduced to only 300 MPa, a 40% reduction.At 100 nm elevation, the stress in the channel is reduced to only 140MPa, more than a 70% reduction in stress. Neither elevation is too largeto achieve using conventional fabrication techniques, but it can be seenthat even a 10 nm elevation will reduce the stress in the channelsomewhat.

As previously described, transverse compressive stress generallydegrades transistor performance for both N-channel and P-channeltransistors, while longitudinal compressive stress generally degradestransistor performance only for N-channel transistors. For P-channeltransistors, longitudinal compressive stress generally improvesperformance. Therefore, it is advantageous to suppress the STI regionsas shown in FIG. 9 in all directions bordering an N-channel diffusion,but it is advantageous to suppress the STI regions only at thelongitudinal borders of P-channel diffusions. At the transverse bordersof P-channel diffusions, it is advantageous to allow the STI elevationto remain equal to or higher than that of the channel regions.

As used herein, the “elevation” of a structure refers to the elevationof its top surface. Note that it is elevation relative to channelregions specifically, rather than the entire diffusion, that is mostsignificant. However, in many fabrication processes it is easier toelevate the entire diffusion region relative to the STI. Elevation of“the channel region”, as used herein, therefore does not precludeelevation of regions larger than the channel region itself, up to andincluding the entire diffusion region of which it is part. Also, it canbe seen in FIG. 9 that the corners of the diffusion region 912 adjacentto the STI regions 914 and 916 have been rounded. This practice isoptional, but suggested in order to minimize undesired concentrations ofelectrons and holes in the corners. When used, the practice results inelevations that are not constant over the entire diffusion region.Therefore, as used herein, it is the average elevation over the entirechannel region that is considered to determine the “channel elevation”.

FIG. 11 illustrates a layout region that includes the four transistors310, 312, 314 and 316 from FIG. 3, as well as four additionaltransistors 1110, 1112, 1114 and 1116 similar to transistors 310, 312,314 and 316 and in a grouping immediately to the right in the drawing.The transistors 1110, 1112, 1114 and 1116 are defined by gate conductors1122 and 1124 crossing P-channel diffusion region 1118 and N-channeldiffusion region 1120. All of the STI regions shown in the drawing aresuppressed, except for the shaded STI regions 1130, 1132 and 1134. STIregion 1130 is longitudinally adjacent to P-channel diffusion region318, and STI region 1132 is longitudinally adjacent to both P-channeldiffusion regions 318 and 1118. STI region 1134 is longitudinallyadjacent to P-channel diffusion region 1118. The STI regions 1130, 1132and 1134 have an elevation that is at least as high as that of thechannel regions of each of the P-channel transistors 310, 312, 1110 and1112. Typically the elevation of these STI regions will be higher thanthat of the channel regions, but it is sufficient that it be at least ashigh as the channel regions as useful for retaining the compressivestress they exert longitudinally on the four P-channel transistors. Thetechnique of suppressing STI regions is beneficial by itself aspreviously described, but it is preferred that the STI regionslongitudinally-adjacent to P-channel diffusions retain a higherelevation than that of other STI regions.

FIG. 11A is a cross-sectional view of the layout region of FIG. 11,taken at sight lines A-A. In this drawing the gate stacks 322, 324, 1122and 1124 are shown symbolically as including the spacers longitudinallybracketing the gate conductors themselves. In this cross-section, onlythe P-channel transistors 310, 312, 1110 and 1112 are shown. It can beseen that the STI regions 1130, 1132 and 1134 longitudinally adjacent tothe P-channel diffusions 318 and 1118 are elevated above the level ofthe diffusion regions 318 and 1118. The elevation is designated as 1142in FIG. 11A, but the amount is unimportant for purposes of the presentdiscussion since it is significant only that the elevation be at leastas high as that of the diffusion regions 318 and 1118. As such, the STIregions exert beneficial compressive stress longitudinally into thediffusions 318 and 1118, including into the channel regions below gatestacks 322, 324, 1122 and 1124.

FIG. 11B is a cross-sectional view of the layout region of FIG. 11,taken at sight lines B-B. As in FIG. 11A, the gate stacks 322, 324, 1122and 1124 in FIG. 11B are shown symbolically as including the spacerslongitudinally bracketing the gate conductors themselves. In thiscross-section, only the N-channel transistors 314, 316, 1114 and 1116are shown. It can be seen that for the N-channel transistors, unlike forthe P-channel transistors, the diffusions 320 and 1120 are elevatedabove the level of the STI regions 1136, 1138 and 1140 longitudinallyadjacent to the diffusion regions 320 and 1120. The elevation of thechannels is designated as 1144 in FIG. 11B, and in various embodimentsthe amount of the elevation can be, for example, 10 nm. Moreadvantageously the elevation is 50 nm, and even more advantageously itis 100 nm. Since the diffusion regions 320 and 1120 are elevated abovethe level of the surrounding STI regions, the compressive stress exertedboth longitudinally and transversely into the diffusions 320 and 1120,and more particularly into the channel regions below gate stacks 322,324, 1122 and 1124, is exerted below the surface where most of thecurrent flows.

FIGS. 12A, 12B and 12C illustrate fabrication steps that can be used toform the P-channel structures of FIG. 11A, and FIGS. 13A, 13B, 13C and13D illustrate fabrication steps that can be used to form the N-channelstructures of FIG. 11B. An embodiment will be described in which bothare formed in a common sequence of steps. It will be understood that theprocess steps actually take place after step 130 (FIG. 1), but the steps(at least to the level of detail shown) are fully anticipated andintended during the layout generation steps up to and including step 124and beyond.

In FIG. 12A, P-channel diffusions 318 and 1118 are formed byconventional techniques. In FIG. 13A, N-channel diffusions 320 and 1120are also formed by conventional techniques.

In FIG. 12B, trenches 1210, 1212 and 1214 are etched into the substratefor STI regions 1130, 1132 and 1134, respectively. Simultaneously, inFIG. 13B, trenches 1316, 1318 and 1320 are etched in to the substratefor STI regions 1136, 1138 and 1140, respectively.

In FIG. 12C, trenches 1210, 1212 and 1214 are filled with an oxide toform the STI regions 1130, 1132 and 1134, respectively. Simultaneously,in FIG. 13C, trenches 1316, 1318 and 1320 are filled with an oxide toform the STI regions 1136, 1138 and 1140, respectively.

Next, a masking material is formed over the surface of the wafer, andopened lithographically over all the STI regions except thoselongitudinally-adjacent to P-channel diffusions. In FIG. 11, the onlySTI regions remaining protected by the etch mask material are STIregions 1130, 1132 and 1134.

Next, the STI regions not protected by the masking material are etchedback, or otherwise reduced in elevation, to the desired level ofsuppression below the wafer surface. For the P-channel transistors, thecross-section remains the same as shown in FIG. 12C. For the N-channeltransistors, the resulting cross-section is as shown in FIG. 13D. As canbe seen in FIG. 13D, the resulting STI regions 1136, 1138 and 1140 aresuppressed below the substrate surface by the elevation differential1144.

It is noteworthy that the elevation reduction of STI is performed beforethe gate stack is applied. Conventionally oxide is overfilled into theSTI trenches and then etched-back or chemically-mechanically polishedback to approximately the level of a superposing masking layer. The gatestack is applied thereafter, and during several oxide etching steps thatare done later, the STI oxide unintentionally may be etched back to alevel below the channel diffusions. But any STI oxide that lies belowthe gate stack is protected from the latter etch-backs. Since the gatestack typically extends beyond the diffusion regions transversely, thegate stack precludes any suppression of the STI, transversely adjacentto the channel regions, to a level below the channel diffusions. So evenif the STI oxide were to be etched-back or polished-back to a levelbelow that of the channel diffusions, it would still be elevated above(or at least as high as) the channel regions at thetransversely-adjacent borders thereof. As previously explained, STI inthose locations applies detrimental compressive stress transversely intothe channel, thereby degrading performance of both N-channel andP-channel transistors.

By contrast, the steps of FIGS. 12A, 12B, 12C, 13A, 13B, 13C and 13D areall performed before the gate stacks are applied. Thus they result insuppression of the STI transversely-adjacent to all the diffusionregions, as well as longitudinally-adjacent to the diffusion regions.Accordingly, after the STI oxide etch-back step, the gate stacks areapplied and the source and drain diffusions are formed, by conventionaltechniques, resulting in the structures shown in FIGS. 11A and 11B.

It will be appreciated that the process steps above, for applying themasking material, opening it over only the desired STI regions, andreducing the elevation of only those STI regions exposed through themasking material, can for many fabrication processes be performed aspart of existing fabrication process steps. For such processes, noadditional steps are required to form the elevation differentials. Inaddition, it will also be appreciated that the formation of elevationdifferentials not only helps to improve device performance, but also cangreatly reduce layout sensitivity of transistor performance.

In an alternative embodiment, the desired elevation differentials areformed by selective silicon epitaxy instead of by over-etching STImaterial.

FIG. 14 is a simplified block diagram of a computer system 1410 suitablefor performing various steps shown in FIGS. 1 and 2. In one embodiment asingle computer system is used for performing all the steps, whereas inanother embodiment different computer systems are used for variousdifferent ones of the steps. Computer system 1410 typically includes atleast one processor 1414 which communicates with a number of peripheraldevices via bus subsystem 1412. These peripheral devices may include astorage subsystem 1424, comprising a memory subsystem 1426 and a filestorage subsystem 1428, user interface input devices 1422, userinterface output devices 1420, and a network interface subsystem 1416.The input and output devices allow user interaction with computer system1410. Network interface subsystem 1416 provides an interface to outsidenetworks, including an interface to communication network 1418, and iscoupled via communication network 1418 to corresponding interfacedevices in other computer systems. Communication network 1418 maycomprise many interconnected computer systems and communication links.These communication links may be wireline links, optical links, wirelesslinks, or any other mechanisms for communication of information. Whilein one embodiment, communication network 1418 is the Internet, in otherembodiments, communication network 1418 may be any suitable computernetwork.

User interface input devices 1422 may include a keyboard, pointingdevices such as a mouse, trackball, touchpad, or graphics tablet, ascanner, a touchscreen incorporated into the display, audio inputdevices such as voice recognition systems, microphones, and other typesof input devices. In general, use of the term “input device” is intendedto include all possible types of devices and ways to input informationinto computer system 1410 or onto computer network 1418.

User interface output devices 1420 may include a display subsystem, aprinter, a fax machine, or non-visual displays such as audio outputdevices. The display subsystem may include a cathode ray tube (CRT), aflat-panel device such as a liquid crystal display (LCD), a projectiondevice, or some other mechanism for creating a visible image. Thedisplay subsystem may also provide non-visual display such as via audiooutput devices. In general, use of the term “output device” is intendedto include all possible types of devices and ways to output informationfrom computer system 1410 to the user or to another machine or computersystem.

Storage subsystem 1424 stores the basic programming and data constructsthat provide the functionality of certain embodiments of the presentinvention. For example, the various modules implementing thefunctionality of certain embodiments of the invention may be stored instorage subsystem 1424. These software modules, when executed byprocessor 1414, perform computer-implemented steps of FIGS. 1 and 2.

Memory subsystem 1426 typically includes a number of memories includinga main random access memory (RAM) 1430 for storage of instructions anddata during program execution and a read only memory (ROM) 1432 in whichfixed instructions are stored. File storage subsystem 1428 providespersistent storage for program and data files, and may include a harddisk drive, a floppy disk drive along with associated removable media, aCD-ROM drive, an optical drive, or removable media cartridges. Thedatabases and modules implementing the functionality of certainembodiments of the invention may be stored by file storage subsystem1428.

Bus subsystem 1412 provides a mechanism for letting the variouscomponents and subsystems of computer system 1410 communicate with eachother as intended. Although bus subsystem 1412 is shown schematically asa single bus, alternative embodiments of the bus subsystem may usemultiple busses.

Computer system 1410 itself can be of varying types including a personalcomputer, a portable computer, a workstation, a computer terminal, anetwork computer, a television, a mainframe, or any other dataprocessing system or user device. Due to the ever-changing nature ofcomputers and networks, the description of computer system 1410 depictedin FIG. 14 is intended only as a specific example for purposes ofillustrating certain embodiments of the present invention. Many otherconfigurations of computer system 1410 are possible having more or lesscomponents than the computer system depicted in FIG. 14.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in this art.The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. A method for forming an integrated device, comprising the steps of:forming a first diffusion well for the device, the first diffusion wellincluding a first channel region for a first transistor, the firstchannel region defining longitudinal and transverse directions for thefirst diffusion well; forming a first STI region transversely adjacentto the first diffusion well, such that the first channel region iselevated relative to the first STI region; forming a first gateconductor overlying the first channel region after the step of forming afirst STI region; and forming first source and drain regions for thefirst transistor in the first diffusion well on opposite sides of thefirst channel region longitudinally.
 2. A method according to claim 1,wherein the first transistor is an N-channel transistor, furthercomprising the step of forming a second STI region longitudinallyadjacent to the first diffusion well, such that the first channel regionis elevated also relative to the second STI region.
 3. A methodaccording to claim 1, wherein the first transistor is a P-channeltransistor, further comprising the step of forming a second STI regionlongitudinally adjacent to the first diffusion well, such that the firstchannel region is not elevated relative to the second STI region.
 4. Amethod according to claim 1, wherein the first transistor is anN-channel transistor, further comprising the steps of: forming a secondSTI region longitudinally adjacent to the first diffusion well, suchthat the first channel region is elevated also relative to the secondSTI region; forming a second diffusion well for the device, the seconddiffusion well including a second channel region for a secondtransistor, the second transistor being a P-channel transistor, thesecond channel region defining longitudinal and transverse directionsfor the second diffusion well; forming a third STI region transverselyadjacent to the second diffusion well, such that the second channelregion is elevated relative to the third STI region; forming a fourthSTI region longitudinally adjacent to the second diffusion well, suchthat the second channel region is not elevated relative to the fourthSTI region.
 5. A method according to claim 4, wherein the steps offorming first, second, third and fourth STI regions collectivelycomprise the steps of: forming respective first, second, third andfourth depressions for the first, second, third and fourth STI regions;filling each of the first, second, third and fourth depressions with STImaterial to an elevation at least as high as the first and seconddiffusion regions; applying a masking layer over at least the STIregions, the masking material exposing the first, second and third STIregions but protecting the fourth STI region; and reducing the elevationof only those STI regions exposed through the masking layer.